it will be seen that the
upper (positive) carbon of the open-arc emits most of the light. Thus
most of the light tends to be sent downward, but the lower carbon
obstructs some of this with a resulting dark spot beneath the lamp.

The gas-mantle followed closely after the arrival of the carbon arc and
is responsible for the existence of gas-lighting on the streets at the
present time. It is a large source of light and therefore its light
cannot be controlled by modern accessories as well as the light from
smaller sources, such as the arc or concentrated-filament lamp. As a
consequence, there is marked unevenness of illumination along the
streets unless the gas-mantle units are spaced rather closely. Even with
the open-arc, without special light-controlling equipment there is about
a thousand times the intensity near the lamps when placed on the corners
of the block as there is midway between them.

In 1879 the incandescent filament lamp was introduced and it began to
appear on the streets in a short time. It was a feeble, inefficient
light-source, compared with the arc-lamp, but it had the advantage of
being installed on a small bracket. As a consequence of simplicity of
operation, the incandescent lamp was installed to a considerable extent,
especially in the suburban districts.

[Illustration: THE MOORE NITROGEN TUBE

In lobby of Madison Square Garden]

[Illustration: CARBON-DIOXIDE TUBE FOR ACCURATE COLOR-MATCHING]

[Illustration: MODERN STREET LIGHTING

Tunnels of light boring through the darkness provide safe channels for
modern traffic]

The open-arc lamp possessed the disadvantage of emitting a very unsteady
light and of consuming the carbons so rapidly that daily trimming was
often necessary. In 1893 the enclosed arc appeared and although it
consumed as much electrical energy as the open-arc and emitted
considerably less light, it possessed the great advantage of operating a
week without requiring a renewal of carbons. By surrounding the arc
by means of a glass globe, little oxygen could come in contact with the
carbons and they were not consumed very rapidly. The light was fairly
steady and these arcs operated satisfactorily on alternating current.
The latter feature simplified the generating and distributing equipment
of the central station.

The magnetite or luminous arc-lamp next appeared and met with
considerable success. It was more efficient than the preceding lamps but
was handicapped by being